1. Field of the Invention
The present invention relates to a high-frequency heating apparatus including an inverter power supply as the power source for performing dielectric heating by using magnetron such as a microwave oven. The invention also relates to a cooling construction of a magnetron-driving power supply of the high-frequency heating apparatus. The present invention further relates to a power control method for the safe operation of a high-frequency power supply apparatus and for the overheat protection of the detection performance of a cooking detection sensor.
2. Description of the Related Art
A magnetron-driving power supply, in which a high-voltage circuit, a low-voltage circuit, and a leakage transformer are integrated on a printed circuit board and which is a so-called inverter circuit, has been widely used as a microwave oven power supply. FIG. 6 is a front and side view of a prior-art magnetron-driving power supply.
As such, respective components are integrally mounted on one board and provided as one unit board. 1 denotes a leakage transformer, and 2 denotes a radiating fin for cooling a power switching element. FIG. 7 is a block diagram showing an inverter circuit.
Voltage from a commercial power supply is converted to a unidirectional voltage by a unidirectional power supply portion 3 composed of diode bridges. The unidirectional voltage undergoes current smoothing and voltage smoothing by a rectifying filter 6 composed of a choke coil 4 and a smoothing capacitor 5. Output from the rectifying filter 6 is converted to high-frequency power of 30-50KHz by an inverter portion 7. As a method for this inverter portion, various styles including a voltage resonant type, a current resonant type, a partial resonant type, and a half-bridge method are applicable. The electric power is converted to high-frequency high-voltage by the leakage transformer 1. This high-frequency voltage is converted to a direct current high voltage by means of a high-voltage rectifying means 8 composed of a capacitor and a diode.
The leakage transformer 1 includes tertiary winding, supplies electric power to a filament of a magnetron 9 through a high-voltage lead wire 10, and makes electrons radiate from a cathode. On the other hand, the voltage which has been converted to a direct current high voltage by the high-voltage rectifying means 8 is applied, similarly through the high-voltage lead wire 10, to an anode-cathode section of the magnetron 9 and radiates a microwave output into a microwave oven, thereby heating food by dielectric heating. Moreover, the inverter portion 7 is controlled by an inverter control portion 11, and a power switching element inside the inverter portion 7 is ON/OFF controlled. A magnetron-driving power supply 12 is constructed by the above construction. In addition, 13 is connected to a chassis to be a ground potential.
Next, FIG. 6 is a front and side view of a magnetron-driving power supply 12, which have been already described, and 14 denotes a power switching element, which is screwed on the radiating fin 2 to be closely fitted. Loss of this power switching element 14 is conducted as heat to the radiating fin 2 and cooled by forcedly cooling air together with the radiating fin 2. 15 denotes a high-pressure capacitor and 16 denotes a high-pressure diode, thereby constructing a high-pressure rectifying means 8. All of these components are mounted on a paper phenol board 17, thus constructing the integrated magnetron-driving power supply 12, which has been described for FIG. 7. As this inverter method, a dual transistor method is assumed, therefore, two power switching elements 14 are provided.
The radiating fin is constructed by a group of fins laterally protruding from a core portion which is in the parallel direction with respect to the transistor, and is cooled by air passing therethrough. The power switching element 14 is connected to the radiating fin 2 with heat-conductive silicone grease or the like sandwiched therebetween and transmits heat to the radiating fin 2. When the fin group of the radiating fin 2 is sufficiently exposed to the air, it is unnecessary to use a very expensive low-loss power switching element as a power switching element 14. Also, the magnetron-driving power supply 12 is completed in this condition, therefore, it is unnecessary to prepare an assortment of various models according to the setting type and extremely unified and efficient manufacturing can be realized.
In a magnetron-driving power supply using this inverter circuit, it is very common that cooling of the respective components is performed by forced cooling by the cooling fan. FIG. 8 is a cooling constructional view of a prior-art magnetron-driving power supply. 18 denotes a cooling fan, which is activated by a motor 19. In addition, 20 denotes an orifice of the cooling fan. In addition, 21 denotes an air guide for guiding air from the cooling fan to a magnetron-driving power supply 12. The air from the cooling fan is centralized by this air guide 21 so as to contact the magnetron-driving power supply 12. FIG. 9 is a cooling constructional view in a case of transmission through the air guide 21. The most significant theme is cooling the radiating fin 2 connected to the leakage transformer 1 and a power switching element 14 whose temperature is most intensive. FIG. 10 is a view showing airflow with a prior-art cooling construction. When a propeller fan is used as a cooling fan, outflow of the air from the cooling fan becomes radial in the direction A. Accordingly, the air A from the cooling fan, first, contacts a part of the air guide 21-P then flows in the direction Axe2x80x2 of the drawing. Accordingly, the air velocity of Axe2x80x2 considerably decreases compared to the initial air A flowed out from the cooling fan. Moreover, air B which is directly parallel with the axial direction of the cooling fan directly contacts the magnetron-driving power supply, however, the air force is weak compared to that of the air A flowing out radially. Accordingly, although the air of the cooling fan cannot be effectively utilized perfectly, an advantage such as a simple cooling construction exists, therefore, such a cooling construction has been conventionally used.
However, in recent years, with strong demand for a higher microwave oven output, power consumption of a magnetron-driving power supply, in particular, power consumption of a switching element and a leakage transformer has become remarkably high. Accordingly, it has become necessary to design a more efficient cooling construction for a magnetron-driving power supply. In addition, in order to deal with downsizing and novel designs of microwave ovens, restrictions on a machine room space for a cooling system, a magnetron-driving power supply, etc., have became stricter and an efficient cooling construction within a limited space has become necessary.
Such a high-frequency heating apparatus using an inverter power supply as the power source is disclosed, for example, in Japanese Unexamined Patent Publication No. Hei-6-50550. FIG. 14 and FIG. 15 show the high-frequency heating apparatus according to the prior art disclosed in the above-mentioned publication.
FIG. 14 is a cross sectional view showing a power supply section in the high-frequency heating apparatus. Numeral 31 indicates a heating chamber. Numeral 32 indicates a magnetron for generating high-frequency power. Numeral 33 indicates an inverter power supply for supplying power to the magnetron. Numeral 34 indicates a cooling fan for cooling the power supply section.
FIG. 15 is a perspective view showing the appearance of the inverter power supply 33. Numeral 35 indicates a semiconductor rectifier element for rectifying the commercially available electric power. Numeral 36 indicates a semiconductor switching element for converting the rectified power into high-frequency power. Numeral 37 indicates a high-voltage transformer for converting the high-frequency power into high voltage and thereby supplying the power to the magnetron 32. Numeral 38 indicates a heat radiation fin to which the semiconductor rectifier element 35 and the semiconductor switching elements 36 are attached.
Nevertheless, in the prior art configuration, the power supplied to the magnetron is supplied through the semiconductor rectifier element 35 and the semiconductor switching elements 36 by inverter scheme. These semiconductor devices have low heat resistivity, and have a disadvantage in that the devices suffer thermal damage easily at a temperature exceeding the heat resistivity limit even for a short period of time. Accordingly, for example, in a case where the cooling fan 34 has stopped due to a failure, the semiconductor rectifier element 35 and the semiconductor switching elements 36 have suffered thermal damage in a short period of time due to self-heat generation.
In some high-frequency heating apparatuses according to the prior art, the magnetron is provided with a temperature switch or the like. When the cooling fan motor fails, an abnormal temperature rise is detected in the magnetron, whereby the operation of the high-frequency heating apparatus is terminated. Nevertheless, in a high-frequency heating apparatus using an inverter power supply, the semiconductor rectifier element and the semiconductor switching elements suffer thermal damage before the abnormal temperature rise is detected in the magnetron. Thus, such a temperature switch does not have the effect of protecting the inverter power supply.
Moreover, in the configuration of a prior art high-frequency heating apparatus, for example, an automatic cooking detection sensor apparatus cannot start automatic cooking when the temperature environment exceeds the limit at the start of cooking, in order to secure performance which would not be secured above the temperature limit. Further, in a case where the temperature environment exceeds the limit during automatic cooking, the cooking is not properly completed. On the other hand, the power supply apparatus comprises means for preventing troubles caused by overheating, however, in a general configuration, the power supplying is forced to terminate by mechanical means during cooking in order to prevent damage to the components due to overheating.
In the above-mentioned configuration according to the prior art there has been a problem wherein when the temperature of the power supply apparatus or the temperature of the cooking detection sensor exceeds the temperature limit for securing function and performance during the supplying of high-frequency power, the power supplying is suddenly terminated by mechanical means even during cooking, and that reason for the suspension in heating is not displayed. Another problem has been that power control cannot be carried out in a simple configuration for detecting temperature changes of varied temperature limit levels.
The present invention for solving the above-mentioned problems aims to provide a high efficient cooling system within a limited space and a high-frequency heating apparatus including the cooling system. The invention also aims to provide a high-frequency heating apparatus monitoring temperature of an inverter power supply and a mainly components, controlling an output of the inverter power supply according to the monitored information, and avoiding breaking of an inverter power supply caused of over-heating. The invention further aims to provide a high-frequency apparatus performing stability in unusual temperature and displaying a fault defect when the heating apparatus cannot cock caused by temperature factor.
In order to solve the above-mentioned problems, the present invention provides a cooling system for a magnetron-driving power supply including a magnetron-driving power supply having those on a printed board, a radiating fin formed by an aluminum extrusion molding method and mounted a power switching element having an IGBT and a such thereon, an inverter portion, an inverter controlling portion for controlling the inverter portion, a leakage transfer for booting a high-frequency alternating voltage, and a high voltage rectifying means for applying direct current high voltage to a magnetron connected to a secondary winding of the leakage transfer, a cooling fan for forcedly cooling the magnetron-driving power supply, and an air guide for guiding air form the cooling fan to the magnetron-driving power supply. In the system, the inverter portion turns on/off electric power of a commercial power supply power at high speed by means of the power switching element and converting the electric power to a high-frequency alternating voltage. The radiating fin cools the power switching element by dispersing loss, which occurs in a closely bound manner with the power switching element. The system is structured so that the axial direction of the cooling fan intersects the printed wiring board of the magnetron-driving power supply at an acute angle, and one end of an opening portion of the air guide is latched with the cooling fan.
Thus, it becomes possible to efficiently contact air from the cooling fan against the magnetron-driving power supply via an air guide, thereby improving cooling efficiency.
In order to resolve the above-mentioned problem, the present invention also provides a high-frequency heating apparatus including an semiconductor rectifier element and semiconductor switching element, both of them are major components of the inverter power supply, characterized in low heat resistivity but a high heat generation rate, and attached a temperature sensors thereto. The heating apparatus also includes a controlling means having a microcomputer for monitoring the temperature of the temperature sensors and for controlling the output of the inverter power supply according to the monitored information.
By the above-mentioned structure of the high-frequency heating apparatus, in a case where an abnormal temperature rise is caused in the semiconductor rectifier element and the semiconductor switching elements for any reason, the output of the inverter power supply is reduced or terminated before the temperature reaches the heat resistivity limit of the components. Accordingly, thermal damage in the inverter power supply is avoided.
In order to resolve the above-mentioned problems, the present invention further provides a high-frequency apparatus including a power supply control means for separately setting a power supply control start temperature setting level for securing performance of the automatic cooking detection sensor due to a temperature factor, a power supply control start temperature setting level for overheat protection of the power supply apparatus against self-heat generation, and a power supply control start temperature setting level for overheat protection of the power supply apparatus during heating when the temperature of the cooling air is high at the start of cooking. Then, when the temperature detected by a temperature sensor reaches the respective temperature limit levels, the high-frequency output is controlled in order to secure normal performance.
By the process of the separately setting above-mentioned, the automatic cooking sensor does not stop during cooking, whereby normal detection performance is secured. Even in cooking by manual time setting other than automatic sensor cooking, the high-frequency output is controlled before the power supply apparatus suffers overheat damage whereby damage is prevented in any change in the cooling temperature environment.
At the start of cooking by automatic cooking sensor detection, even in a case where the cooling temperature environment is so high that the automatic cooking sensor detection cannot be carried out normally owing to the high temperature, the condition is notified to the user. Accordingly, the condition is notified to the user even when completion of automatic sensor cooking is delayed, or when cooking is temporarily stopped.